This invention relates to apparatus for detecting the presence of sheets at a predetermined location along the feed path of a document counter, endorser or the like.
Often it is desirable to have a signal indicating the presence of a sheet at a predetermined location along a feed path. Such a sheet signal may be used for counting, length measurement, jam detection and the like.
Various methods of sheet detection have been used, each having its own particular drawbacks. Mechanical sensors using fingers have the disadvantage that the fingers may interfere with the movement of the sheet, especially if the sheet is thin or has become worn.
Optical methods are also commonly used. One common expedient is to direct a light beam across the sheet feed path to a detector so that the beam is interrupted during the passage of the sheet. Another expedient is to sense a beam of light reflected back from the sheet. Although these methods do not interfere with sheet movement, they depend on the opacity or reflectance of the sheets for the production of a sheet signal, so as to be unreliable if these optical properties of the sheets are atypical. Thus, certain documents may be of a color that does not reflect light of the incident wavelength, and thus fail to be detected by a reflectance measurement. Other documents may be so thin as to miss being detected by an opacity measurement. Both of these techniques are also susceptible to error if the documents in question contain holes. If the portion of the document being scanned contains a hole, it will generally be interpreted as two separate documents.